This invention relates to an eddy current probe configured to accurately measure a distance between the eddy current probe and a surface of an object spaced apart from the eddy current probe. In particular, this invention relates to an improved or modified eddy current probe (MEC) that may be used in groups of two or more to measure the distance to multiple locations on a surface of an object, including objects formed from a low conductivity material, and are configured to operate without undesirable noise and cross-talk caused by coupling of the electro-magnetic fields of closely spaced eddy current probes.
Known eddy current probes are configured to monitor and analyze rotating and reciprocating machinery and measure the distance between an eddy current probe and a surface of an object. Typically, an eddy current probe is positioned near a monitored target object such as a rotating shaft of a machine or an outer race of a rolling element bearing. Known eddy current probes are designed to monitor and/or measure the distance to the surface of high conductivity materials such as metals. Further, known eddy current probes operate at frequencies within the range of about 500 KHz to about 1 MHz.
Carbon fiber composite materials, such as graphite are used in a variety of structures, including carbon brushes for motors, seals and bearings, vane pump components, assemblies for fuel pumps, starters, and alternators, and bi-polar plated for fuel cells. Additionally, graphite structures are used in rotating devices wherein higher bandwidths and sub-mil resolution are required. There is therefore a need in many industries to measure the distance to the surfaces of graphite structures and to locate cracks or defects in the surfaces of graphite structures. Industries with such a need include the power, aircraft, automotive, turbine, mass transit, and railway industries.
Known sensor technology, including eddy current sensors, capacitive sensors, magnetic sensors, and acoustic sensors, are not able to measure the distance to low conductivity materials, such as graphite. Further, these known sensors are not able to monitor and analyze, such as for inspection and balancing, rotating components formed from low conductivity materials, such as graphite.
In some applications, there is a need to measure the distance to multiple locations on the surface of the object. In this situation, two or more eddy current probes may be used. However, the signals from adjacent eddy current probes may undesirably couple, resulting in inaccurate distance measurements.
Thus, it would be desirable to provide an improved structure for an eddy current probe that allows two or more of the eddy current probes to be used together to accurately monitor, analyze, and measure the distance between the probes and a surface of an object, including an object formed from a low conductivity material such as graphite, without undesirable noise and cross-talk caused by coupling of the electro-magnetic fields of adjacent eddy current probes.